One of the smallest cities in Tarrant County is home to one of its biggest workforces.Fidelity Investments, which is celebrating its 30th anniversary in Texas this year, now has more than 4,500 employees working at its 337-acre regional campus in Westlake, a town with only about 1,000 residents. Another 400 people work at Fidelity facilities in Irving and Grapevine.When the campus first opened in Westlake in 2001, Fidelity consolidated facilities from Las Colinas and elsewhere in North Texas. Since then, the campus has added jobs through a mix of internal expansion and relocations from its Boston headquarters and other sites.The latest influx has come since 2010, when a second building opened in Westlake. Construction on that building began in 2008 but was mothballed for 18 months during the economic meltdown. But since it was completed, about 100 workers a month have been added, said Scott Orr, Fidelity’s vice president of public affairs in Westlake. Overall, about a quarter of the Westlake jobs have been moved to Texas from other Fidelity sites.On a recent visit to the campus, set back off busy Texas 114 in the wooded calm of the former Circle T Ranch, officials showed us the complex’s spacious cafeteria, open seating area and bright modern work spaces. There are a fitness center inside and walking trails outside.About 1,700 employees work the phones day and night to help individual investors and retirement plan customers. But this is not your average call center. These “financial services associates” get six months of training for jobs that pay a starting salary of $50,000 a year, said Tom Herrick, senior vice president and regional site leader.Another 1,100 people work in corporate technology positions. Other business groups with employees in Westlake include asset management, fund accounting, corporate finance, legal and audit, making it a microcosm of the whole company, said Roger Stiles, another regional site leader and chief investment officer for Fidelity Personal Investing.The Westlake complex is now Fidelity’s largest regional operations center outside New England. Overall, Fidelity has 41,000 employees.